The present invention relates to a method for synchronising communication of framed data via asynchronous base stations in a cellular communications system, e.g. a CDMA-system (Code Division Multiple Access). The synchronisation method is performed continuously, but in particular at connection establishment and during execution of soft handover.
The invention is also directed to an arrangement for performing the above mentioned method.
Today there is an increasing interest in using CDMA or spread spectrum systems in commercial applications. Some examples include digital cellular radio, land mobile radio, satellite systems, and indoor and outdoor personal communications networks referred to herein collectively as cellular systems.
CDMA allows signals to overlap in both time and frequency. Thus, CDMA signals share the same frequency spectrum. In the frequency or the time domain, the multiple access signals appear to be on top of each other.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are high. This is a result of the properties of a wide band CDMA system, such as improved interference diversity, voice activity gating, and reuse of the same spectrum in interference diversity.
In principle, in a CDMA system the informational data stream to be transmitted is superimposed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, providing a bit stream. One way to generate this signature sequence is with a PN-process (pseudo-noise) that appears random, but can be replicated by an authorised receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or xe2x88x921. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique spreading code. The ratio between the signature sequence bit rate and the information bit rate is called the spreading ratio.
A plurality of coded information signals modulate a radio frequency carrier, for example by QPSK (Quadrature Phase Shift Keying), and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. If the receiver is authorised, then the composite signal is correlated with one of the unique codes, and the corresponding information signal can be isolated and decoded.
In CDMA, also referred to as DS-CDMA (direct sequence-CDMA) to distinguish it from FH-CDMA (frequency hopping-CDMA), the xe2x80x9cinformation bitsxe2x80x9d referred to above can also be coded bits, where the code used is a block or convolutional code. One or more information bits can form a data symbol. Also, the signature sequence or scramble mask can be much longer than a single code sequence, in which case a sub-sequence of the signature sequence or scramble mask is added to the code sequence.
In a CDMA cellular communications system, each cell has several modulator-demodulator units or spread spectrum modems. Each modem consists of a digital spread spectrum transmit modulator, at least one digital spread spectrum data receiver and a searcher receiver. Each modem at the base station BS can be assigned to a mobile station as needed to facilitate communications with the assigned mobile station MS. In many instances many modems are available for use while other ones may be active in communicating with respective mobile stations. A soft handover scheme is employed for a CDMA cellular communications system in which a new base station modem is assigned to a mobile station while the old base station modem continues to serve the call. When the mobile station is located in the transition region between the two base stations, it communicates with both base stations. Similarly, if one base station is responsible for more than one geographical sector handover may be carried out between different sectors belonging to the same base station.
When mobile station communications are established with a new base station or a new sector, for instance, the mobile station has good communications with the new cell or sector, the old base station/modem discontinues serving the call. This soft handover is in essence a make-before-break switching function. The mobile station determines the best new base station, or sector, to which communications are to be transferred to from an old base station, or sector. Although it is preferred that the mobile station initiates the handover request and determines the new base station, handover process decisions may be made as in conventional cellular telephone systems wherein the base station determines when a handover may be appropriate and, via the system controller, request neighbouring cells, or sectors, to search for the mobile station signal. The base station receiving the strongest signal as determined by the system controller then accepts the handover.
In the CDMA cellular communications system, each base station normally transmits a pilot carrier signal in each of its sectors. This pilot signal is used by the mobile stations to obtain initial system synchronisation and to provide robust time, frequency and phase tracking of the base station transmitted signals during a so called air interface chip synchronisation phase. The RNC (Radio Network Control node) maintains its synchronisation with the PSTN (Public Switched Telephone Network).
An active set for a specific mobile station is a listing of sectors via which the mobile station communicates. Adding and/or dropping sectors from the active set is called an ASU (active set update). Thus, a regular handover from a first base station (serving a first sector) to a second base station (serving a second sector) can be defined as the active set before handover containing only the first sector and after the handover containing only the second sector. Handover from the first to the second base station may, of course, also be defined as the active set originally containing several sectors i.a. the first sector, but not the second sector and after handover the active set containing several sectors i.a. the second sector, however not the first sector. Furthermore a handover may be performed either between identical frequencies, a so called intra radio frequency handover (intra RF-HO) or between different frequencies, a so called inter radio frequency handover (inter RF-HO). The exact definition of handover is nevertheless irrelevant for the present application, since the invention only concerns active set update and in particular adding one or more sectors to the active set.
The active set. may also be different for the up- and the downlink connection for a particular mobile station. For instance, it is possible that the active set contains many different sectors of one and the same base station for the uplink and only one of these sectors for the corresponding downlink connection.
During macro diversity the active set contains sectors, which are served by more than one base station. Macro diversity must be used during a soft handover, while a hard handover implicates that the active set never contains more than one sector during the procedure.
Radio frequency synchronisation is accomplished through detection and selection of a particular chip sequence, which is associated with the strongest radio frequency carrier received by the mobile station. This allows identification of the xe2x80x9cbest servingxe2x80x9d base station. Said chip sequence is referenced to a system time that is used, for instance, to set the air interface frame transmit time.
In a CDMA system, overlap of time-slots as in TDMA (Time Division Multiple Access) systems is not a problem since a mobile station transmits continuously, and thus does not need to synchronise to other mobile stations. However, when a mobile station is connected to more than one base station in macro-diversity, there is a need to synchronise the base stations in the downlink (also known as the forward link).
Macro-diversity in a CDMA system can be achieved with synchronised base stations. The base stations are usually synchronised with all base station""s digital transmissions being referenced to a common CDMA system-wide time scale that uses the GPS (Global Positioning System) time scale, which is traceable to and synchronous with UTC (Universal Coordinated Time). The signals from all the base stations are transmitted at the same instant.
In order to enable macro-diversity, the base stations can be synchronised as described above through a common time reference; GPS. Therefore, the signals transmitted from the base stations are synchronised in time. However, due to different propagation delays in the links, the signals arrive at different time instants at the mobile station. Normally in CDMA systems a rake receiver is used to handle time dispersion and the macro-diversity can be seen as time dispersion from the receivers point-of-view. The principle of the rake receiver is to collect the energies from different paths and combine them before a bit-decision is made.
Methods for continuously monitoring parameters of delay between two nodes in an ATM or frame relay network are known from U.S. Pat. No. 5,450,394. Special measurement cells contain a time stamp indicating the time a cell is sent and a delay value, which indicates a difference between reception and transmission times.
The document U.S. Pat. No. 4,894,823 discloses an alternative method for time stamping data packets, which are transmitted through a fixed communications network. Delays experienced by the data packets in network nodes are measured by inserting an originate time value in the header of each packet upon entering a node and updating this time value in an exit time stamp function when the packet has been transported through the node.
A method for time alignment of transmissions over downlinks in a CDMA system is disclosed in WO, A1, 94/30024. Signals for a specific cellular call connection are synchronised through firstly, a mobile station measuring the time difference between the connected base station""s signal and a macro-diversity candidate base station""s signal. This measurement is secondly transmitted to the network, which finally compensates for the difference and synchronises the base stations so that a handover may be performed where no data is lost during the procedure.
U.S. Pat. Nos. 5,450,394 and 4,894 823 provide solutions for estimating transmission delays in framed data communications systems. However, the documents do not teach how to achieve synchronised communication between multiple base stations and a specific mobile station in spite of these delays.
According to WO, A1, 94/30024 a method is known for accomplishing time alignment of transmissions over downlinks in a CDMA system. Nevertheless, there is no solution to how these transmissions should be controlled when the delay differences between signals transmitted from different base stations exceed the duration of one half data frame.
An object of the present invention is thus to minimise the synchronisation error between information frames which are sent to a specific mobile station from two or more asynchronous base stations or sectors. By asynchronous is here meant that a phase difference is permitted between signals transmitted from at least two different base stations and that the clock units in different base stations are not locked to each other.
Another object of the invention is to avoid having to rely on an external time reference receiver in each asynchronous base station in order to meet the synchronisation requirements during update of the active set for a mobile station.
Another object of the invention is to minimise the need for buffering in asynchronous base stations which simultaneously receive information frames from a specific mobile station.
A further object of the invention is to relax the buffering needs in mobile stations and thereby reduce the complexity of the mobile stations.
Yet a further object of the invention is to minimise the average round-trip delay experienced in a cellular radio communications system and in a CDMA communications system in particular. By round-trip delay is here meant the total time it takes (on average) for a hypothetical message to be sent from one end point of a connection to the other and back again.
These objects are met by the present invention by generating certain system frame counter states in a central node in the systemxe2x80x94a radio network control nodexe2x80x94being connected to one or more base stations. Corresponding local frame counter states are generated in each base station in the system. A current sample of the system frame counter state is regularly sent out from the radio network control node to its connected base stations, in order to synchronise each local frame counter with the system frame counter state, which functions as a frame numbering reference within the cellular radio communications system.
According to one aspect of the present invention there is provided a method to regularly send a system frame counter state from a central node to its connected base stations. Each of the base stations adjust their local frame counter states, so that they are all aligned with the system frame counter state. Synchronisation of data packets being communicated via the base stations is then accomplished by sending one data packet per data frame, which is numbered in accordance with a frame counter state. The frame counter states are in the uplink leg of a connection generated locally in each base station and in the downlink leg of the connection, the frame counter states are derived from the system frame counter states in the central node, which is typically a radio network control node.
The above method is hereby characterised by what is apparent from claim 1.
According to another aspect of the present invention there is provided a method for establishing a connection between a particular mobile station and at least one base station, which is based on the synchronisation method above. First, an active set, comprising at least one downlink and one uplink channel, is defined for the mobile station. The base station(s) at which such channels shall be allocated, is(are) determined by pilot signal strength measurements performed by the mobile station. Generally, all sectors whose pilot signal strength value exceeds a predetermined threshold are candidates for the active set. Nevertheless, a downlink channel need not necessarily be allocated in all those sectors and no more than one uplink channel need ever to be allocated. Second, a timing advance value is set for each downlink channel in the active set. The timing advance value specifies an offset between a common downlink control channel for the sector and the downlink channel in question, and is chosen to a value which results in the most uniform distribution of the transmission load on the network and radio resources in the system, in respect to the connections already in progress. Each base station measures, at regular intervals, a common downlink control channel offset between its local frame counter states and the common downlink control channel for each of its sectors. The results of the measurements are reported to the central node. As a third step, a downlink channel offset is calculated by adding the common downlink control channel offset to the timing advance value. Finally, a specific frame number is assigned to each data frame on each respective downlink channel. The frame number indicates in which data frame a particular data packet, that is received from the central node, shall be transmitted. The data frames are numbered according to following. An initial data frame, starting the downlink channel offset value after the current state of the local frame counter state, is given a frame number equal to the current state of the local frame counter. The local frame counter is, on average, incremented at a tick rate which corresponds to one tick per the duration of a data frame. However, due to adjustments of the local frame counter according to updates from the system frame counter state the local frame counter may temporarily have a tick rate, which is either slightly higher or slightly lower than one tick per the duration of a data frame. Subsequent data frames are allocated frame numbers according to their order in relation to the initial data frame.
A method for establishing a connection according to this aspect of the invention is hereby characterised by what is apparent from claim 10.
According to a further aspect of the present invention there is provided a method for commencing communication, via at least one second sector, with a particular mobile station which is already communicating information via at least one first sector, by utilising the synchronisation method above. First, a frame offset between a downlink channel in the active set and a common downlink control channel of a candidate sector for an ASU is measured by the mobile station. Second, the frame offset value is reported to a central node. Third, the second sector is added to the active set. Fourth, a timing advance value and a downlink channel offset value for a downlink channel in the second sector is calculated. Fifth, the offset between the data frames to be transmitted on the downlink channel in the second sector and the common downlink control channel for this sector is set equal to the timing advance value. Finally, a specific frame number is given to each data frame on the downlink channel in the second sector. This is carried out by assigning an initial data frame, which starting from the local frame counter state in the base station serving the second sector plus the downlink channel offset value, falls within half the duration of a data frame a frame number equal to the following local frame state in the base station serving the second sector. Each subsequent data frame is then allocated an integer incrementation the initial number, which is equal to the order of each respective data frame in relation to the initial data frame.
A method for commencing communication via an additional sector, when already communicating via a first sector, according to this aspect of the invention is hereby characterised by what is apparent from claim 11.
An arrangement according to the invention for communicating framed information in a cellular radio communications system comprises one or more central nodes plus one or more base stations. The central node, which is typically a radio network control node, comprises in its turn a master timing unit, a master control unit and a diversity handover unit. The master timing unit generates system frame counter states, which are sent out to the base stations, that are connected to the central node. The master control is a general control unit for the central node. This unit, for instance, determines when to perform an ASU. Furthermore, it calculates timing advance values and downlink channel offset values, which are utilised when numbering data frames on downlink channels. The diversity handover unit is responsible for handling simultaneous communication with a mobile station, via more than one base station.
The above mentioned arrangement of the invention is hereby characterised by what is apparent from claim 22.
The present invention thus offers a solution for performing an active set update (e.g. in connection with soft handover execution) in a cellular radio communications system comprising asynchronous base stations, without demanding GPS-receivers in any base station.
The proposed solution also ensures synchronisation during connection establishment to an asynchronous base station.
Such small synchronisation errors result in low average round-trip delays in the system and allow the transport connections between the radio network control node and the base stations to be asynchronous, e.g. ATM connections.
It also guarantees that there will be no frame slip errors neither in the downlink nor in the uplink of a connection. Moreover the demands for buffering can be relaxed in the base stations as well as in the mobile stations.
As a consequence of the low buffering demand mobile stations can be made less complex and with simpler rake receivers.